Some twenty years ago, the selective in vitro toxicity of the 2-deoxyguanosine analog, 9-β-D-arabinofuranosylguanine or AraG for T lymphoblasts was noted by several investigators. AraG is metabolized in a unique fashion by deoxyguanosine kinase and incorporated into mitochondrial DNA (FIG. 2.4). These observations led to the synthesis of a more water-soluble AraG prodrug, 2-amino-6-methoxypurine arabinoside (506U, Nelarabine), for potential clinical application in the treatment of T lymphoblastic diseases. This compound, developed over a number of years by Glaxo, is now FDA-approved for the treatment of relapsed T cell ALL and T cell lymphoblastic lymphomas.